I have been doing a lot of research myself on the single tier stand! I also purchased the design build (brutus ten) from BYO magazine. They recommend a 2 inch tubing, mostly for support if you plan on mounting the gas lines and liquid lines to the frame(the holes drilled will weaken a thinner tube) And for the thickness they recommend an 11 gauge or .120" I purchased the plans for a local welder and they only cost me $3.00 + shipping. I know its not the sabco design, but its a nice setup!
2 inch tubing @ almost an 1/8" is overkill, imho - unless you are not going to have a center support. I think most designs have a center support (and it makes a great place to mount the pumps to).
Mine is 1/16" X 1.5". Plenty sturdy, and light enough for me to muscle around (with the aid of wheels on two of the legs - a la wheel barrow).